1. Field of the Invention
Embodiments of the present invention generally relate to marine seismic surveying and, more particularly, to a method for eliminating surface related multiples from a record of seismic data.
2. Description of the Related Art
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources, which generate seismic waves, and seismic receivers, which detect seismic waves. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer. The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices that record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying typically utilizes seismic energy sources and seismic receivers located in the water, which are either towed behind a vessel or positioned on the water bottom from a vessel. The energy source is typically an explosive device or compressed air system that generates seismic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves reflects back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. The seismic receivers typically used in marine seismic surveying are pressure sensors, such as hydrophones. Additionally, motion sensors, such as geophones or accelerometers may be used. Both the sources and receivers may be strategically repositioned to cover the survey area.
Seismic waves, however, reflect from interfaces other than just those between subterranean formations, as would be desired. Seismic waves also reflect from the water bottom and the water surface, and the resulting reflected waves themselves continue to reflect. Waves that reflect multiple times are referred to as multiple reflections or “multiples”. Surface multiples are those waves that have reflected multiple times between the water surface and any upward reflectors, such as the water bottom or formation interfaces, before being sensed by a receiver. Generally, surface multiples are considered undesirable noises that interfere with and complicate the desired signal. Considerable effort is expended in the design of seismic data acquisition and the processing of seismic data to limit the impact of multiple reflections on the final processed seismic profiles. Even so, in many areas, the quality of seismic data is lowered, sometimes substantially, by the presence of multiple reflections.
Various prior art methods have been tried for removal or elimination of surface multiples from recorded traces. If the subsurface reflectors are flat or have dips in one direction, then a one-dimensional data acquisition geometry may provide sufficient information for the surface multiples to be predicted using a two-dimensional multiple prediction algorithm. However, if the subsurface reflectors have dips in more than one direction or the data acquisition geometry is not one-dimensional (e.g., due to ocean currents), then surface multiples may only be accurately predicted by a three-dimensional multiple prediction algorithm, which is generally more costly than a two-dimensional multiple prediction algorithm.
Accordingly, a need exists in the art for a method for determining when it would be necessary to perform a two-dimensional multiple prediction algorithm versus a three-dimensional multiple prediction algorithm to avoid unnecessary costly expenditure.